Browsing by Author "Lee, HC"

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Calculations of the inter-subband scattering rates of electrons in GaAs/AlGaAs quantum wells
(2003-05-01)
Lee, HC
;
Sun, KW
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
In this presentation, we report the calculated results on hot electron relaxation through inter- and intra-subband scatterings with LO phonons in quantum well structures. The scattering rates were calculated for electrons excited both in the gamma valley and L valleys. The types of optical phonons adapted in our model are determined based on the dielectric continuum model. We have also studied the dependence of the scatterings on the structure parameters of the quantum well. (C) 2003 Elsevier Science Ltd. All rights reserved.
Inter- and intra-subband relaxation of hot electrons in GaAs/AlGaAs quantum wells
(2003-05-01)
Sun, KW
;
Huang, CL
;
Huang, GB
;
Lee, HC
;
電子工程學系及電子研究所
In this work, we have studied the inter- and intra-subband scattering of hot electrons in quantum wells using the hot electron-neutral acceptor luminescence technique. We have observed direct evidence of the emission of confined optical phonons by hot electrons excited slightly above the n = 2 subband in GaAs/Al0.37Ga0.63As quantum wells. Scattering rates of photoexcited electrons via inter- and intra-subband LO phonon emission were calculated based on the dielectric continuum model. We found that, for wide wells with the Al composition of our experiments, both the calculated and experimental results suggest that the scattering of the electrons is dominated by the confined LO phonon mode. In the calculations, scatterings among higher subbands are also dominated by the same type of phonon at well width of 10 nm. (C) 2003 Elsevier Science Ltd. All rights reserved.
Investigation of electron-optical phonon interactions in moderate wide InxGa1-xAs/GaAs strained quantum wells
(2001-12-01)
Lin, SD
;
Lee, HC
;
Sun, KW
;
Lee, CP
;
電子工程學系及電子研究所
In this presentation, we have calculated the electron-optical phonon scattering rate of GaAs/AlAs quantum wells and average electron energy loss rate as a function of well width of the GaAs/AlxGa1-xAs quantum wells. We have also studied the Raman and hot electron-neutral acceptor luminescence in moderate wide InxGa1-xAs/GaAs strained quantum wells (with 10 nm in well width and 30 nm in barrier width) to determine the dominant phonon mode emitted by the hot electrons in the wells at 15 K. The hot electron-neutral acceptor luminescence spectrum of the strained quantum well sample shows an oscillation period of about 22 meV which indicates that the hot electrons relaxed mostly through emissions of the InAs confined phonons. (C) 2001 Elsevier Science B.V. All rights reserved.
Phase diagram of crystals of dusty plasma
(1997-10-01)
Lee, HC
;
Chen, DY
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Rosenstein, B
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電子物理學系
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Department of Electrophysics
Dust particles effectively charged by plasma recently have been optically observed to exhibit crystalline phases not expected of Wigner or Yukawa crystals. Under varying conditions the crystal sometimes appears as deformed and oriented three-dimensional close-packed lattices of bcc, fcc, or hcp type, but mostly as a triangular array of vertical chains of particles. The unusual phases are shown to be caused by dipole-dipole interactions. The dipole moments are induced on the dust particles by gravity and by drag forces generated by ion stream. We describe in detail stable lattice structures and present the highly complex phase diagram of the dusty plasma. It turns out that in large parts of the phase diagram the stable phases indeed correspond to chains, but particles in neighboring chains belong to different sublattices. The stability of the lattices against excitations due to compression (i.e., aspect ratio variations) and vibration (i.e., phonons or charge density waves) is established.
Power saving of a dynamic width controller for a monolithic current-mode CMOS DC-DC converter
(2005)
Lee, HC
;
Chang, KT
;
Chen, KH
;
Chen, WT
;
電控工程研究所
We propose the dynamic power MOS width controlling technique and the adaptive gate driver voltage technique to find out the better approach to power saving in DC-DC converters. It demonstrates that the dynamic power MOS width controlling technique has much improvement in power consumption than that of the adaptive gate driver voltage technique when the load current is heavy or light. After the dynamic power MOS width modification, the simulation results show that the efficiency of current-mode DC-DC buck converter can be improved from 92% to about 98% in heavy load and from 15% to about 16.3% in light load. However, the adaptive gate driver voltage technique has only little improvement of power saving. It means that the dynamic width controller is the better approach to power saving in the DC-DC converter.
Significance of dimensionality and dynamical screening on hot carrier relaxation in bulk GaAs and quantum wells
(2003-11-01)
Lee, HC
;
Sun, KW
;
Lee, CP
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
We have found theoretically that the reduced dimensionality from bulk GaAs to quantum wells has a strong effect on hot carrier relaxations at highly excited carrier densities. The distinct density of state and the dynamical screening cause hot carriers in quantum wells to relax significantly slower than in the bulk when the carrier density is above a critical value of 2 x 10(18) cm(-3). With the random phase approximation, the dynamical screening in quantum wells appears to be much stronger than that in the bulk and as important as the hot phonon effect at high carrier densities. We also found that the dependence of the average energy-loss rates on the well width in quantum wells becomes more appreciable when Al compositions are high. (C) 2003 Elsevier Ltd. All rights reserved.
Structure effects on electron-optical phonon interaction in GaAs/AlxGa1-xAs quantum wells
(2002-07-01)
Lee, HC
;
Sun, KW
;
Lee, CP
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Based on the dielectric continuum model, we have studied the electron-optical phonon scattering rates in GaAs/AlxGa1-xAs quantum wells with different structure parameters. It was found that the scattering rate of the symmetric interface phonon mode has a stronger dependence on the Al composition in the barriers than that of the confined mode. The effective phonon energy emitted by hot electrons in GaAs/AlxGa1-xAs quantum wells with various Al compositions was estimated and the calculated value agrees with the experimental results qualitatively. For the dependence on the well width, scattering rates of the S+ mode drop considerably as the well width is increased. The dependence of the electron-optical phonon interaction on structure parameters can be clearly explained by the H and G factors defined in the article. (C) 2002 American Institute of Physics.
Structure effects on inter- and intra-band scattering of electrons in GaAs/AlxGa1-xAs and strained InxGa1-xAs/GaAs quantum wells
(2003)
Lee, HC
;
Sun, KW
;
Lee, CP
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Based on the dielectric continuum model, we have studied the dependence of electron-optical phonon scattering rates in GaAs/AlxGa1-xAs quantum wells with different structure parameters. It was found that the dependence of scattering rates of symmetric interface mode on Al composition in the barrier was stronger than that of the confined mode. The average phonon energy emitted by hot electrons in GaAs/AlxGa1-xAs quantum wells with various Al composition was estimated and the calculated value agrees with the experimental results qualitatively. For the dependence on the well width, scattering rates of the S+ mode dropped considerably as the well width is increased. The hot electron-neutral acceptor luminescence spectrum of the strained InxGa1-xAs/GaAs quantum well sample shows an oscillation period of about 22 meV which indicates that the hot electrons relaxed mostly through emissions of the InAs confined phonons.